Playing AP Mel build, your default rush is Luden's Companion. In cases, when an enemy team is full of tanks and HP stackers, go Liandry's Torment.
Adapt to an enemy team. Are they full of tanks, and they rush tons of Magic Resist? You rush Void Staff as #2 or #3 item to counter MR stacking. Their team has champs with lots of healing? Rush Oblivion Orb as #2 item (upgrade to Morellonomicon as 4th or 5th item).
Somebody is going after you each fight? Always getting ulted by enemy assassins? Rush Zhonya's Hourglass.
Tips and Tricks
Riot
Mel can reflect enemy projectiles, including powerful spells. Wait until she casts Rebuttal before throwing powerful projectiles her way.
The more Mel hits you, the more Overwhelm stacks she applies. If your health drops too low her next hit will kill you, so back off for a couple seconds to let the Overwhelm stacks fall off.